The present invention relates to magnetic direction sensing systems and particularly those for use in vehicles.
U.S. Pat. No. 4,953,305, assigned to the present assignee, discloses a magnetic field sensor and microprocessor-controlled compass system for a vehicle. The system utilizes flux-gate magnetic sensors to sense the magnitude of the earth's magnetic field in two channels of measurement. The sensor data, if plotted on an X-Y Cartesian coordinate plane, would be as shown in FIG. 1. For a properly calibrated compass, the plot of sensor data creates a perfect circle centered around the origin of the coordinate plane when the vehicle travels in a 360.degree. loop, as indicated by graph A of FIG. 1. The radius of the circle represents the detected earth's magnetic field strength, and the vehicle's compass heading at a particular time during travel is represented by a point on the circle. By calculating the angle at which the point forms with the X-Y coordinate plane, the compass heading of the vehicle may be determined. As is known, depending on the location of the vehicle, the detected magnitude of the earth's magnetic field can vary significantly.
The sensed magnetic field will also be affected if there is a change in vehicular magnetism. Changes in the magnetism of a vehicle can be caused by, for example, driving the vehicle near the electrical power feeders of train or subway systems, installing a magnetic cellular antennae on the vehicle's roof, parking under an AC powerline, or even driving through a car wash, which can flex the sheet metal in the vicinity of the compass sensor and change its magnetic characteristics. Such a change in vehicular magnetism will cause the magnetic field sensed by the compass channels when the vehicle is heading in a given direction to be either greater than or less than that expected for a vehicle with no magnetic interference. As a result, the plot of sensor data will be shifted away from the origin of the coordinate plane in some direction, resulting in a pattern such as the circle shown as graph 13 of FIG. 1 when the vehicle travels a 360.degree. loop. The magnitude of the shift of sensor data from the origin is proportional to the magnitude of the change in vehicular magnetism.
The compass system of the above-mentioned patent provides automatic and continuous calibration to account for changes in the vehicle's magnetism, and thus the system's reaction to the earth's magnetic field during the life of the vehicle. The calibration system includes means for testing the data received from the compass sensor to determine the maximum and minimum signal levels during movement of the vehicle through a completed 360.degree. path of travel however circuitous the path may be. This data is averaged over several such paths of vehicular travel to provide continuously updated and averaged compensation correction information. The automatic and continuous calibration is capable of correcting the compass system when the plot of sensor data experiences small shifts away from the origin of the coordinate plane due to small drifts in vehicular magnetism. The origin of the coordinate plane in these circumstances is still contained within the circle plotted when the vehicle travels a 360.degree. loop, and the crossings of the sensor data on the axes of the coordinate plane are used to calculate the spans of the signal levels along each axis which determine the center of the circular plot of sensor data. Compensation signals are then generated based on the difference between the center of the circle and the origin of the coordinate plane. However, if the shift of sensor data is large enough such that the origin of the coordinate plane is not contained within the circular plot of sensor data created when the vehicle travels a 360.degree. loop, then heading information cannot be calculated and the calibration system cannot provide correction in this somewhat unusual situation unless the sensor data experiences a subsequent shift that causes the origin of the coordinate plane to again be contained. Because such a subsequent shift may never occur or, if it does, may occur only after an undesirably long period of time, the compass system of the above-mentioned patent provides means to reinitiate calibration in these situations.
Reinitiation of calibration involves the collecting and centering of spans of sensor data followed by the collecting and centering of two circles of sensor data, which causes the origin of the coordinate plane to coincide with the center of the circular plot of sensor data. As such, the reinitiation process enables the compass system to recover from any change in vehicular magnetism and to provide accurate heading information. In order to detect situations where reinitiation of the calibration system is desirable, it is known to have the compass system maintain saturation limits at the outer boundaries of the range of measurement of the sensor data. For 8-bit sensor data, these saturation limits are at 0 and 255, as shown in FIG. 1. If a large change in vehicular magnetism causes the sensor data to shift and the current data is plotted outside of these limits for a continuous period of five minutes, then calibration is restarted. Such a shift is shown by graph C of FIG. 2, with the dashed portion thereof indicating the range of heading directions of the vehicle that would cause the sensor data to remain outside of the saturation limits. However, intermediate changes in vehicular magnetism are possible which, while causing the plot of sensor data to shift and to not contain the origin of the coordinate plane when the vehicle completes a 360.degree. loop, do not cause the sensor data to be plotted outside of the saturation limits. Such a shift is shown by graph D of FIG. 3. As such, it is known to also provide for a reinitiation of calibration if 15 ignition cycles of at least 5 minutes duration are completed without obtaining a crossing point on the axes of the X-Y coordinate plane. Furthermore, it is known to enable the operator of the vehicle to manually reinitiate calibration by operating a switch, button, or the like. Manual reinitiation would most likely occur when the operator notices that the displayed heading information is erroneous for an extended period of time. Although the above-mentioned means by which to cause reinitiation of calibration enables the compass system to ultimately recover from changes in vehicular magnetism of any magnitude, such reinitiation is considered a rather extreme measure since it requires the clearing of all prior sensor readings and calibration data. Thus, until sufficient data is collected to calibrate the system, the system operates in an uncalibrated state.
Although this system is a substantial improvement in vehicle compass operation and provides more accurate heading information over differing operating conditions, its somewhat lengthy averaging process and method of gradual compensation makes it primarily suited for the compensation of slow and gradual changes in vehicular magnetism. As such, this compass system may be unable to adequately compensate for and recover from an abrupt and significant change in vehicular magnetism caused by, for example, driving the vehicle near the electrical power feeders of train or subway systems. Thus, such an event may cause a substantial impairment of compass operation resulting in at least short-term erroneous heading information to be displayed until recalibration or reinitialization of the system is achieved.
Further, a particular problem with vehicular magnetism exists before sale of a new vehicle to a customer. At this time, the vehicle may be substantially magnetized due to either the manufacturing process or the method of delivery of the vehicle to the dealer. In order to ensure that the compass system is providing accurate heading information upon initial power-up by the customer, changed or existing vehicular magnetism must be compensated for or eliminated. The means chosen to perform this function should be easy and efficient so that servicing of the vehicle is avoided and should be capable of being performed either at the factory or at individual dealerships. Although factory compensation of a new vehicle's compass has been standard practice for many years, current methods have proven to be inadequate. For example, eliminating the magnetism requires special degaussing equipment that is very expensive, and assigning the duty of manual calibration to the manufacturer or to individual dealers is problematic. One method of compass compensation at the factory involves identifying the magnetic field at a particular location and, when the vehicle is positioned in a predetermined direction at this location, providing calibration signals to correct for any differences in the displayed heading and the known heading for the existing magnetic field at said position along the assembly line. This method is problematic in that a magnetically stable location may be impossible to maintain in a factory environment due to the possibility of stray or changing magnetic fields and disturbances which would potentially cause miscalibration of the compass resulting in erroneous heading information being displayed.
The compass system disclosed in commonly assigned U.S. Pat. No. 5,737,226, entitled VEHICLE COMPASS SYSTEM WITH AUTOMATIC CALIBRATION, issued on Apr. 7, 1998, operates substantially similar to that in the above-described U.S. Pat. No. 4,953,305, except that it utilizes a modified control program that calibrates the compass system utilizing only two averaged data points and one raw data point obtained from travel of the vehicle in less than a complete closed loop for purposes of calibrating the compass system during initialization following manufacture and at such times that the compass system determines that the obtained sensor data falls outside the saturation thresholds that previously required reinitialization of the compass system. Thus, the compass system disclosed in U.S. Pat. No. 5,737,226 allowed the compass system to become calibrated much more quickly following manufacture and to recover more quickly when the sensor data is detected as being outside the saturation threshold limits.
The manner by which the compass system disclosed in U.S. Pat. No. 5,737,226 recalibrates itself by identifying the center of a circular plot of data is described below with reference to FIG. 4. When a vehicle makes a slight turn, the data obtained from the sensors may take the form of the arc shown when plotted relative to Cartesian coordinates. The starting point T of the arc shown corresponds to the output of the sensors obtained just prior to the vehicle starting the turn. As the vehicle makes a turn, intermediate raw data points, such as point U, are read from the sensors. At the completion of the turn, the data point derived from the sensor output signals would correspond to ending point V. To perform a calculation of the center W of the arc (or center of circle F), it is desirable that starting point T and ending point V are data points in which there is a fair to high level of confidence in their accuracy. Such confidence in the data points may be present when the sensor outputs remain at a constant level for a predetermined period of time as would be the case when the vehicle is traveling straight. The center W of the plotted arc is calculated by assuming a predetermined value for the radius r and identifying the two points that are a distance r from both starting point T and ending point V. To discriminate between the two points thus obtained, an intermediate raw data point U is referenced, since the true center point will be that which is farthest away from intermediate point U.
To ensure at least a minimal amount of accuracy, the compass system will not recalibrate using two data points that are less than a predetermined distance a away from one another. This predetermined distance criterion represents a trade-off between accuracy and rapid calibration. Because the disclosed compass system subsequently utilizes the averaged data obtained using the calibration technique disclosed in U.S. Pat. No. 4,953,305, the sacrifice of accuracy only temporarily affects the compass system.
Although the above compass system solves some of the problems associated with the compass system disclosed in U.S. Pat. No. 4,953,305, it does not increase the speed at which the compass system compensates for less significant changes in vehicular magnetism. For example, so long as the signals from the sensors do not exceed the saturation threshold but nevertheless exhibit a change in vehicular magnetism, the compass system disclosed in U.S. Pat. No. 5,737,226 would rely upon the calibration technique disclosed in U.S. Pat. No. 4,953,305, whereby the center of the new circle would not be obtained until the vehicle travels through a complete new 360.degree. loop. Further, in such an event, the center of the new circle would be averaged with that of the prior circle thereby further slowing down the responsiveness of the compass system to such abrupt and less significant changes in vehicular magnetism.
Because compass systems employing magneto-inductive sensors do not require the use of the analog-to-digital converters utilized by compass systems having flux-gate or magneto-resistive sensors, magneto-inductive compass systems are not confined by the saturation limits or dynamic range of an analog-to-digital converter. Thus, if the calibration technique disclosed in U.S. Pat. No. 5,737,226 were implemented in a magneto-inductive compass system, the sensor outputs would never exceed in a saturation threshold and thus the quick calibration technique employing two points of data would not be used even when the changes in vehicular magnetism are significant.
U.S. Pat. No. 4,807,462 also discloses a compass system that recalibrates itself using data obtained during such time that the vehicle travels in less than a complete 360.degree. loop. As illustrated in FIG. 5, the compass system disclosed in this patent calculates the center of a circle utilizing three data points. In particular, this compass system determines the coordinates (X.sub.0, Y.sub.0) of the center of the circle by determining the coordinates of the intersection point of perpendicular bisectors of lines drawn between the first and second data points and between the second and third data points. This compass system recalculates the center of the circle each time a new data point is detected. When a new data point is detected, the compass system utilizes this new data point with the two most recent of the prior three data points. Thus, this system continuously calibrates using just three data points. Because this compass system does not establish any criterion for accepting data points used to calculate the center of the circle, and because this system apparently throws out the previously calculated center point each time a new center point is calculated, the system is not very accurate. For example, if a newly obtained data point is offset from the prior circle due to travel across railroad tracks, the compass system will become improperly calibrated based upon this one inaccurate data point.
Thus, there exists a need for a compass compensation system capable of more accurately compensating for and recovering from abrupt changes in vehicular magnetism regardless of the significance of the change and having the capability to more efficiently and more accurately compensate for the initial vehicular magnetism of a new vehicle.